Engine air/fuel ratio sensing device

ABSTRACT

An engine air/fuel ratio sensing device for measuring the oxygen partial pressure or concentration in the exhaust gas of an engine. The device has a sensor and a control circuit coupled to each other. The sensor consists of an electrolyte oxygen pump cell and an electrolyte oxygen sensor cell, both cells having a gap portion therebetween. The control circuit provides a pumping current through a current limiting resistor to the pump cell and measures two terminal voltages across the current limiting resistor to derive the actual pump current and the internal resistance of the pump cell, the latter being converted into the actual temperature to which the sensor is exposed. The actual pump current and the actual temperature are used to correct the actual pump current to a proper pump current dependent on the temperature, in accordance with a predetermined equation.

BACKGROUND OF THE INVENTION

This invention relates to a device for measuring an oxygen concentrationwithin an exhaust gas from an internal combustion engine, etc., to sensethe air/fuel (hereinafter abbreviated as A/F) ratio and in particular toan improved engine A/F ratio sensing device of an oxygen pump typeconstructed using an ion conducting solid electrolyte.

It is hitherto well known in the art to control e.g., an, engine of anautomobile to run at a stoichiometric (theoretical) A/F ratio by sensingits combustion state in relation to the stoichiometric A/F ratioaccording to the variation of an electromotive force produced by thedifference of the oxygen partial pressure between the exhaust gas andthe atmosphere by means of an oxygen sensor constructed with an ionconducting solid electrolyte such as stabilized zirconia.

When the A/F ratio which is given by the weight ratio of air to fuel isthe stoichiometric A/F ratio of 14.7, the above type oxygen sensor canprovide a large output variation, while outside the stoichiometric A/Fratio it provides a substantially null output variation. Therefore, whenthe engine is operated at an A/F ratio outside the stoichiometric A/Fratio, the output of such an oxygen sensor can not be utilized.

Thus, an A/F ratio sensor of an oxygen pump type which eliminates such adisadvantage and enables the engine to be operated at any A/F ratio hasalready been proposed. However, such a sensor is not practical for thereason of an outstanding variation of its characteristic due totemperature variation.

FIG. 1 shows an arrangement of an A/F ratio sensing device of an oxygenpump type, and FIG. 2 shows a cross sectional view of the sensor in FIG.1 taken along line II--II, which is disclosed in a related applicationSer. No. 606,926 filed May 4, 1984.

In FIG. 1, within an exhaust pipe 1 of an engine (not shown) an A/Fratio sensor, generally designated by a reference numeral 2, isdisposed. This sensor 2 is formed of a solid electrolyte oxygen pumpcell 3, a solid electrolyte oxygen sensor cell 4, and a supporting base5. The solid electrolyte oxygen pump cell 3 includes an ion conductingsolid electrolyte (stabilized zirconia) 6 in the form of a plate with athickness of about 0.5 mm having platinum electrodes 7 and 8 disposed onthe respective sides thereof. The solid electrolyte oxygen sensor cell4, similar to the pump cell 3, includes an ion conductive solidelectrolyte 9 in the form of a plate having platinum electrodes 10 and11 disposed on the respective sides thereof. The supporting base 5supports the oxygen pump cell 3 and the oxygen sensor cell 4 so thatthey are oppositely disposed having a minute gap "d" of about 0.1 mmtherebetween.

An electronic control unit 12 is electrically coupled to the pump cell 3and the sensor cell 4. More specifically, the electrode 10 is connectedthrough a resistor R1 to the inverting input of an operational amplifierA, the non-inverting input of which is grounded through a DC referencevoltage source V. This DC reference voltage serves to control the outputvoltage of the sensor cell 4 to assume said voltage V according to theoxygen partial pressure difference between those within the gap andoutside the gap. The electrode 7 is connected through a resistor Rs tothe emitter of a transistor Tr whose collector is grounded through a DCpower source B and whose base is connected to the output of theoperational amplifier A and the inverting input of the operationalamplifier A through a capacitor C. The electrodes 8 and 11 are grounded.

U.S. Pat. No. 4,272,329 discloses the principle of an A/F ratio sensingdevice of an oxygen type.

In operation, when the oxygen partial pressure within the gap portionbetween the cells 3 and 4 is the same as the oxygen partial pressureoutside the gap portion, the sensor cell 4 generates no electromotiveforce. Therefore, the inverting input of the operational amplifier Areceives no voltage and, accordingly, the operational amplifier Aprovides as an output therefrom a maximum voltage corresponding to thereference voltage V to the base of the transistor Tr. Therefore, thetransistor Tr is made conductive to cause a pump current Ip to flowthrough the electrodes 7 and 8 of the pump cell 3 from the voltagesource B. Then the pump cell 3 pumps oxygen present in the gap portion"d" into the exhaust pipe 1. As a result, the sensor cell 4 develops anelectromotive force "e" thereacross according to the oxygen partialpressure difference on both sides of the cell 4.

Therefore, the oxygen sensor cell 4 applies the electromotive force "e"generated across the electrodes 10 and 11 to the inverting input of theoperational amplifier A through the resistor R1. The operationalamplifier A provides an output now proportional to the differencebetween the electromotive force "e" and the reference DC voltage Vapplied to the non-inverting input. The output of the operationalamplifier A drives the transistor Tr to control the pump current Ip.

Thus, the electromotive force "e" approaches the reference voltage V.Accordingly, the control unit 12 reaches an equilibrium state and servesto provide a pump current Ip necessary for keeping the electromotiveforce "e" at the predetermined reference voltage V. The resistor Rsserves to provide an output corresponding to the pump current Ipsupplied from the DC power source B as a pump current supply means. Thepump current Ip corresponds to an A/F ratio value. This pump current Ipis converted into a voltage by the resistor Rs and is sent to a fuelcontrol unit (not shown) so that the fuel control unit may be controlledat a desired A/F ratio. The resistance of the resistor Rs is selected soas to prevent the pump current Ip from flowing excessively from the DCpower source B. The capacitor C forms an integrator associated with theoperational amplifier A and serves to make the electromotive force "e"precisely coincident with the reference voltage V.

One example of the static characteristics of an A/F ratio sensing deviceof an oxygen pump type thus constructed in the form of a negativefeedback control is shown in FIG. 3. A solid line indicates acharacteristic of pump current Ip as a function of A/F ratio when theA/F ratio sensor 2 is exposed at 600° C. while a dotted line indicates acharacteristic at 800° C. It is found that such a characteristicvariation gives rise to the thermal variation of the ion conductivity ofthe ion conducting solid electrolytes 6 and 9, respectively forming thesolid electrolyte oxygen pump cell 3 and the solid electrolyte oxygensensor cell 4. Experiments reveal that as the temperature of the A/Fratio sensor is varied over a temperature range of an engine exhaustgas, the current Ip, flowing through the oxygen pump cell 3,corresponding to the same A/F ratio varies up to several ten percentagesthereof, resulting in an unpractical A/F ratio sensor.

On the other hand, FIG. 4 shows a temperature dependency of theelectrical resistance of the ion conducting solid electrolytes 6 and 9.Since this characteristic is common to various solid electrolytes, theapplication of this characteristic shown in FIG. 4 enables thetemperature variation of the A/F ratio sensor to be corrected.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide an A/F ratiosensing device of an oxygen pump type, free from the effect of such athermal characteristic variation as above noted, by the detection of thethermal characteristic variation of the A/F ratio sensor per se from thevariation of its internal resistance and by the correction of same.

In order to accomplish this object, the present invention provides anair/fuel ratio sensing device for an engine comprising an air/fuelsensor means having a gap portion for introducing an exhaust gas of theengine, a solid electrolyte oxygen pump cell for controlling an oxygenpartial pressure within the gap portion, and a solid electrolyte oxygensensor cell for producing an electromotive force corresponding to thedifference between the oxygen partial pressure in the exhaust gas withinthe gap portion and the oxygen partial pressure in the exhaust gasoutside the gap portion; and a control circuit means (12'), coupled tothe sensor means, having a current amplifier means for supplying to theoxygen pump cell a pump current necessary for keeping the oxygen partialpressure at a predetermined value, and a resistor (Rs) inserted in theelectrical path through which the pump current flows for limiting thepump current to a predetermined value, the control circuit means furthercomprising: a voltage measuring means, connected to the resistor, formeasuring and providing as outputs therefrom the terminal voltage acrossthe resistor and the terminal voltage across the oxygen pump cell, acomputing means, connected to the voltage measuring means forcalculating from the terminal voltage across the resistor and theresistance of the resistor a first value (Ip) corresponding to the pumpcurrent and for calculating from the first value and the terminalvoltage across the oxygen pump cell a second value (R) corresponding tothe internal resistance of the oxygen pump cell, a temperatureconverting means, connected to the computing means, for producing athird value (T) corresponding to the temperature of the oxygen pump cellin accordance with a predetermined relationship including the secondvalue as an input parameter, and a calibrating means, connected to thecomputing means and the temperature converting means, for providing asan air/fuel signal therefrom a fourth value (Ipo) obtained bycalibrating the first value in accordance with a predeterminedrelationship including the first value and the third value as aparameter.

The temperature converting means preferably comprises a storage meansfor storing a plurality of typical values of the second value as well asnumerical values respectively corresponding to the typical values of thesecond value, and a computing means for retrieving two of the numericalvalues respectively corresponding to two of the typical valuessandwiching the value of the second value, and for calculating anumerical value, as the third third value, corresponding to the secondvalue by means of an interpolating operation from the two numericalvalues. Also, the temperature converting means may calculate the thirdvalue (T) from a converting equation T=B/1n(AR) by using the secondvalue (R), A and B being constants.

The calibrating means preferably comprises a storage means for storing aplurality of typical values representative of the first and third valuesas well as numerical values respectively corresponding to thecombinations of the first and third values, and a computing means forretrieving four of the numerical values respectively corresponding tothe combinations of respective two typical values sandwiching therespective values of the first and third values and for calculating anumerical value, as the fourth value, corresponding to the combinationof the first and third values by means of an interpolating operationfrom the four numerical values. The calibrating means may calculate thefourth value (Ipo) from a calibrating equation Ipo=Ip(To/T)^(C) by usingthe first value (Ip) and the third value (T), To and C being constants.The calibrating means may calculate the fourth value (Ipo) from acalibrating equation Ipo=Ip{1-C(T-To)/To} by using the first value (Ip)and third value (T), To and C being a constant. The constant C ispreferably in the range of about 0.75-1.0.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an arrangment of an A/F ratio sensing device of an oxygenpump type described in a related application Ser. No. 606,926;

FIG. 2 shows a cross-sectional view of the sensor in FIG. 1, taken alongline II--II;

FIG. 3 shows characteristic curves of the sensing device in FIG. 1wherein the ordinate axis denotes a pump current Ip and the abscissaaxis denotes an A/F ratio;

FIG. 4 shows characteristic curves of the sensing device in FIG. 1wherein the ordinate axis denotes temperature T and the abscissa axisdenotes an internal resistance R;

FIG. 5 shows an arrangement of an A/F ratio sensing device of an oxygenpump type, associated with an exhaust pipe, in accordance with oneembodiment of the present invention; and,

FIG. 6 shows data map used in the computing portion 17 in FIG. 5, and

FIG. 7 shows data map used in the computing poortion 18 in FIG. 5.

Throughout the figures, the same reference numerals indicate identicalor corresponding portions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

One preferred embodiment of an A/F ratio sensing device of an oxygenpump type for an engine in accordance with the present invention willnow be described in detail with reference to the accompanying drawings,particularly FIGS. 5 and 6.

In FIG. 5, the difference between the arrangement of this invention andthat of FIG. 1 is the provision of a control unit 12'. Morespecifically, this control unit 12', in addition to the control unit 12having the same arrangement as that shown in FIG. 1, includes an A/Dconverter 13 of a two channel type, and computing portions 14-18. Thetwo inputs of the A/D converter 13 are connected across the resistor Rs.The input of the computing portion 14 is connected to one output of theA/D converter 13 to calculate a terminal voltage v1-v2 across theresistor Rs. The input of the computing portion 15 is connected to theoutput of the computing portion 14 to calculate the pump current Ip bydividing v1-v2 by the resistance Rs. The one input of the computingportion 16 is connected to the other output of the A/D converter 13 andthe other input of same is connected to the output of the computingportion 15 to calculate the internal resistance R of the oxygen pumpcell 3 by dividing the terminal voltage v2 across the oxygen pump cell 3by the pump current Ip. The input of computing portion 17 is connectedto the output of the computing portion 16 to calculate temperature Tfrom the internal resistance R provided as an output from the computingportion 16. The two inputs of the computing portion 18 are respectivelyconnected to the outputs of the computing portions 15 and 17 tocalculate a proper pump current Ipo by correcting the pump current Ipwith respect to the internal resistance R.

In the arrangement of FIG. 5, the terminal voltages v1 and v2 of theresistor Rs are given by the following equations:

    v1=Ip(Rs+R)                                                (1)

    v2=IpR                                                     (2)

The terminal voltages v1 and v2 are respectively converted into digitalsignals by the A/D converter 13 for the facilitation of the computationprocesses, and the digitalized terminal voltages v1 and v2 are providedfor the computing portion 14 which calculates the difference between theterminal voltages v1 and v2. This difference is divided by theresistance Rs in the computing portion 15 whose output therefore assumes(v1-v2)/Rs={Ip(Rs+R)-IpR}/Rs=Ip from equations (1) and (2), resulting inthe calculation of the pump current Ip. In the computing portion 16, theterminal voltage v2 is divided by the output of the computing portion15, i.e., Ip. Therefore, the output of the computing portion 16 assumesv2/Ip=IpR/Ip=R from equation (2), resulting in the calculation of theinternal resistance R of the oxygen pump cell 3. This internalresistance R is converted by the computing portion 17 into thetemperature of the oxygen pump cell 3 which is calculated from therelationship T=B/1n(AR) representing the characteristic shown in FIG. 4,where A and B are constants.

If it is difficult to represent the characteristic curve in FIG. 4 inthe form of a simple equation due to various conditions on the structureof the sensor 2, or if the above logarithm calculation is performed bymeans of a micro-processor etc. which is hard to perform logarithmiccalculations, its is preferable to include in the computing portion 17 amemory means having stored therein map data as shown in FIG. 6 toperform the calculation.

In the calculation of the temperature of the oxygen pump cell 3 by usingthe map data of FIG. 6, the computing portion 17 preliminarily stores ina memory included therein actually measured temperature values T1, T2,T3, ---, Tn, Tn+1, --- respectively corresponding to typical values R1,R2, R3, ---, Rn, Rn+1, --- of the internal resistance R of the pump cell3. Then, when the internal resistance R is supplied to the computingportion 17 from the computing portion 16, temperature values Tn and Tn+1respectively corresponding to typical resistances Rn and Rn+1sandwiching the internal resistance R are retrieved. Then, thecombinations (Rn, Tn) and (Rn+1, Tn+1) of the retrieved values are usedto approximately calculate the temperature T of the pump cell 3corresponding to the resistance R from the equationT={(Tn+1-Tn)/(Rn+1-Rn)}(R-Rn)+Tn. Accordingly, by utilizing only asimple arithmetic operation, this calculation can be easily performed bya micro-processor.

The temperature value T from the portion 17 together with the pumpcurrent Ip from the portion 15 are given to the computing portion 18 inwhich the pump current Ip is corrected, i.e., calibrated to the pumpcurrent Ipo in a reference temperature To. The calibration is made onthe basis of the relationship as described hereinbelow.

When the pump current Ip is controlled so as to keep the electromotiveforce "e" of the oxygen sensor cell 10 at a constant value, therelationships

    e=(RT/4F)1n(Pa/Pv)                                         (3)

    Ip=(4eDA/KTL)(Pa-Pv)                                       (4)

are established. Equation (3) is the famous Nernst's equation wherein Paand Pv respectively designate oxygen partial pressures inside andoutside the gap "d" as parameters representative of A/F ratio. It is tobe noted that R designates the gas constant, F the Faraday's constant,and T temperature. Equation (4) indicates a relationship given at thetime when a rate of oxygen pumped out of the gap "d" by the pump currentIp equilibrates with a rate of oxygen flown in the gap "d" by diffusion,in which e designates the electric charge of electron, D the diffusioncoefficient, A the cross sectional area of the opening of the gap "d", Kthe Boltzmann's constant, and L the effective diffusion path length.From equations (3) and (4), we obtain

    Ip=(4eDA/KTL)Pa                                            (5)

On the other hand, it is known that the diffusion coefficient D of gasis proportional to the 1.75th power of temperature. Therefore, assumingthe proportion constant be G, D=GT¹.75 is given. By substituting this inequation (5), we obtain

    Ip=(4eGA/KL)T.sup.0.75 ×Pa                           (6)

Namely, it is found that the pump current Ip is proportional to theoxygen partial pressure Pa, i.e., A/F ratio and the 0.75 th power oftemperature T. In equation (6), assuming the pump current at thereference temperature To be Ipo, we obtain

    Ipo=(4eGA/KL)To.sup.0.75 ×Pa                         (7)

From equations (6) and (7), we obtain

    Ipo=Ip(To/T).sup.0.75                                      (8)

Equation (8) indicates that the pump current Ip at an arbitrarytemperature T can be calibrated to the pump current Ipo at the referencetemperature To. It is to be noted that various experiments show thatsince equation (8) can not realize a precise calibration due to someeffect caused by the structure of the sensor as well as temperatureununiformity and so on, it is desirable to express

    Ipo=Ip(To/T).sup.C                                         (9)

and to determine the exponent C based on practically measure data. Alsoin the arrangement of FIG. 5, it has been found according to experimentsthat C is preferably in the order of 0.75-1.0. Hence, the computingportion 18 performs the calculation of equation (9) by using the pumpcurrent Ip which is the output of the computing portion 15 and thetemperature T which is the output of the computing portion 17, therebydetermining the pump current Ipo at the reference temperature To.

It is to be noted that if a micro-processor which is hard to performlogarithmic calculations or exponential calculations is used as thecomputing portion 18, the pump current Ipo at the reference temperatureTo may be simply determined by the following equation

    Ipo=Ip{1+C(T-To)/To}

which is a first order equation approximated instead of equation (9).Athough it is hard to apply this equation to an extensive temperaturerange, it is fully practical to use this equation in a usual exhaust gastemperature of an engine.

Also, an interpolating calculation may be made in the computing portion18 by using map data shown in FIG. 7 and stored in a memory in thecomputing portion 18. The abscissa of the map data denotes typicalpoints T1, T2, T3, ---, Tn, Tn+1, --- of the temperature T and theordinate denotes typical points Ip1, Ip2, Ip3, ---, Ipn, Ipn+1, --- ofthe pump current Ip. Each of the cross points of the typical points onthe abscissa and the ordinate indicates the pump current Ipo calibratedcorresponding to the reference temperature To.

In this map data, for example, should the current Ip determined in thecomputing portion 15 be equal to Ip3 and the temperature T determined inthe computing portion 17 be equal to T3, the value Ipo3,3 stored in thememory in the computing portion 18 is outputed as the current Ipo.

Further, for example, should the current Ip determined in the computingportion 15 be a value which is between the tabulated values Ipn andIpn+1, and the temperature T determined in the computing portion 17 be avalue which is between the tabulated values Tn and Tn+1, aninterpolating step is performed in the portion 18 according toconventional methods. Namely, from the combinations of typical point Tnwhich is smaller than and closest to the temperature T and of thetypical points Ipn and Ipn+1 sandwiching the pump current Ip (i.e., thetabulated values closest to the current Ip), two calibrated pumpcurrents(Ipon,n) and (Ipon+1,n) are retrieved by the operation of thecomputing portion 18. From the combination of the retrieved values (Ipn,Ipon,n) and (Ipn+1, Ipon+1,n), a pump current (Ipo,n) is determined bythe interpolating operation due to the fact that the pump current Ip isintermediate between Ipn and Ipn+1. The process of the interpolatingoperation is similar to that described referring to FIG. 6 so that thedescription thereof is omitted.

Then, from the combinations of typical point Tn+1 which is larger thanand closest to the temperature T and of the typical points Ipn and Ipn+1sandwiching the pump current Ip, two calibrated pump currents (Ipon,n+1)and (Ipon+1,n+1) are retrieved by the operation of the computing portion18. From the combination of the retrieved values (Ipn, Ipon,n+1) and(Ipn+1, Ipon+1,n+1), a pump current (Ipo,n+1) is determined by theinterpolating operation due to the fact that the pump current Ip isintermediate between Ipn and Ipn+1. Then, from the combinations oftypical point Tn+1 which is larger than and closest to the temperature Tand of the typical points Ipn and Ipn+1 sandwiching the pump current Ip,two calibrated pump currents (Ipon,n+1) and (Ipon+1,n+1) are retrievedby the operation of the computing portion 18. From the combination ofthe retrieved values (Ipn, Ipon,n+1) and (Ipn+1, Ipon+ 1,n+1), a pumpcurrent (Ipo,n+1) is determined by the interpolating operation due tothe fact that the pump current Ip is intermediate between Ipn and Ipn+1.

Next, from the combination (Tn,Ipo,n) of the above determined pumpcurrent (Ipo,n) corresponding to the temperature Tn and the combination(Tn+1, Ipo,n+1) of the pump current (Ipo,n+1) corresponding to thetemperature Tn+1, the pump current Ipo is calibrated corresponding tothe temperature T by the interpolating operation. It is to be noted thatthis calibrated pump current Ipo has a value calibrated at the referencetemperature To for the pump current Ip, as above described.

The pump current Ipo thus calibrated is used as a signal representativeof an A/F ratio whereby the A/F ratio sensing device of an oxygen pumptype according to this invention can be disposed in the exhaust gas pathwhich is not constant in temperature.

As mentioned above, according to the A/F ratio sensing device of thisinvention, the following excellent advantages are effected:

(1) since the pump current representative of an A/F ratio is calibratedon the basis of the temperature of the A/F ratio sensor, a precise A/Fratio signal having eliminated therefrom characteristic variation due tothermal variation may be always obtained;

(2) since the temperature of the sensor is represented by the internalresistance of the oxygen pump cell forming the sensor, no particularthermal sensor is needed;

(3) the internal resistance of the oxygen pump cell can be easilycalculated by measuring a terminal voltage across a resistor throughwhich the pump current of the oxygen pump cell flows.

Consequently, an engine can be operated at any A/F ratio irrespective oftemperature variation.

It is to be noted that while this invention has been described along theabove embodiment, it should not be limited to the shown and describedembodiment but various modififcations may be made by any one of ordinaryskills in the art without departing from the spirit of this invention.

What I claim as a patent is:
 1. An air/fuel ratio sensing device for anengine comprising an air/fuel sensor means having a gap portion forintroducing an exhaust gas of said engine, a solid electrolyte oxygenpump cell for controlling an oxygen partial pressure within said gapportion, and a solid electrolyte oxygen sensor cell for producing anelectromotive force corresponding to the difference between the oxygenpartial pressure in the exhaust gas within said gap portion and theoxygen partial pressure in the exhaust gas outside said gap portion; anda control circuit means, coupled to said sensor means, having a currentamplifier means for supplying to said oxygen pump cell a pump currentnecessary for keeping said oxygen partial pressure at a predeterminedvalue, and a resistor (Rs) inserted in the electrical path through whichsaid pump current flows for limiting said pump current to apredetermined value, said control circuit means further comprising:avoltage measuring means, connected to said resistor, for measuring andproviding as outputs therefrom the termnal voltage across said resistorand the terminal voltage across said oxygen pump cell, a computing meansconnected to said voltage measuring means, for calculating from saidterminal voltage across said resistor and the resistance of saidresistor a first value (Ip) corresponding to said pump current and forcalculating from said first value and the terminal voltage across saidoxygen pump cell a second value (R) corresponding to the internalresistance of said oxygen pump cell, a temperature converting means,connected to said computing means, for producing a third value (T)corresponding to the temperature of said oxygen pump cell in accordancewith a predetermined relationship including said second value as aninput parameter, and a calibrating means, connected to said computingmeans and said temperature converting means, for providing as anair/fuel signal therefrom a fourth value (Ipo) obtained by calibratingsaid first value in accordance with a predetermined relationshipincluding said first value and said third value as a parameter.
 2. Anair/fuel ratio sensing device for an engine as claimed in claim 1wherein said temperature converting means comprises a storage means forstoring a plurality of typical values of said second value as well asnumerical values respectively corresponding to said typical values ofsaid second value, and a computing means for retrieving two of saidnumerical values respectively corresponding to two of said typicalvalues sandwiching the value of said second value, and for calculating anumerical value, as said third value, corresponding to said second valueby means of an interpolating operation from said two numerical values.3. An air/fuel ratio sensing device for an engine as claimed in claim 2wherein said temperature converting means calculates said third value(T) from the converting equation T=B/1n(AR) by using said second value(R), said A and B being constants.
 4. An air/fuel ratio sensing devicefor an engine as claimed in claim 3 wherein said calibrating meanscomprises a storage means for storing a plurality of typical valuesrespectively of said first and third values as well as numerical valuesrespectively corresponding to the combinations of said first and thirdvalues, and a computing means for retrieving four of said numericalvalues respectively corresponding to the combinations of respective twotypical values sandwiching the respective values of said first and thirdvalues and for calculating a numerical value, as said fourth value,corresponding to the combination of said first and third values by meansof an interpolating operation from said four numerical values.
 5. Anair/fuel ratio sensing device for an engine as claimed in claim 4wherein said calibrating means calculates said fourth value (Ipo) fromthe calibrating equation Ipo=Ip(To/T)^(C) by using said first value (Ip)and third value (T), said To and C being constants.
 6. An air/fuel ratiosensing device for an engine as claimed in claim 5 wherein said constantC is in the range of about 0.75-1.0.
 7. An air/fuel ratio sensing devicefor an engine as claimed in claim 4 wherein said calibrating meanscalculates said fourth value (Ipo) from the calibrating equationIpo=Ip{1-C(T-To)/To} by using said first value (Ip) and third value (T),said To and C being constants.
 8. An air/fuel ratio sensing device foran engine as claimed in claim 7 wherein said constant C is in the rangeof about 0.75-1.0.
 9. An air/fuel ratio sensing device for an engine asclaimed in claim 1 wherein said calibrating means comprises a storagemeans for storing a plurality of typical values respectively of saidfirst and third values as well as numerical values respectivelycorrepsonding to the combinations of said first and third values, and acomputing means for retrieving four of said numerical valuesrespectively corresponding to the combinations of respective two typicalvalues sandwiching the respective values of said first and third valuesand for calculating a numerical value, as said fourth value,corresponding to the combination of said first and third values by meansof an interpolating operation from said four numerical values.
 10. Anair/fuel ratio sensing device for an engine as claimed in claim 1wherein said calibrating means calculates said fourth value (Ipo) fromthe calibrating equation Ipo=Ip(To/T)^(C) by using said first value (Ip)and third value (T), said To and C being constants.
 11. An air/fuelratio sensing device for an engine as claimed in claim 1 wherein saidcalibrating means calculates said fourth value (Ipo) from thecalibrating equation Ipo=Ip{1-C(T-To)/To} by using said first value (Ip)and third value (T), said To and C being constants.